← MCAT®

1D. Principles of Bioenergetics and Fuel Molecule Metabolism

Every carbon and every ATP accounted for: glycolysis, the cycle, beta-oxidation, and the proton gradient that physically turns ATP synthase.

26 topics · 103 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

1D. Free Energy, Keq and Spontaneous Reactions

  1. How it worksHow do enthalpy and entropy trade off inside delta G to decide which way a reaction goes?
  2. How it worksWalk me through how the equilibrium constant and the standard free energy change encode one fact.
  3. Worked exampleFind the standard free energy change for a reaction whose equilibrium constant is 1000 at 25 degrees C.
  4. Common mistakeWhy doesn't a negative delta G mean the reaction will actually happen any time soon?
Your turneasy1 of 3
A reaction has an equilibrium constant of 3485 at 298 K. What is its standard free energy change, in kilojoules per mole? Use R = 8.314 joules per mole per kelvin.

1D. Coupled Reactions and Le Chatelier's Principle

  1. How it worksHow does a shared intermediate let two reactions add their free energy changes together?
  2. Common mistakeWhy isn't a coupled reaction just two separate reactions happening near each other?
  3. Going furtherWhat happens to a reaction's direction when its product is continuously pulled away?
Your turneasy1 of 3
An enzyme couples an unfavorable step whose standard free energy change is plus 20 kilojoules per mole to ATP hydrolysis at minus 43 kilojoules per mole, through a shared intermediate. What is the overall standard free energy change, in kilojoules per mole?

1D. ATP Hydrolysis and Phosphoryl Group Transfer

  1. How it worksHow do charge repulsion and resonance stabilization make ATP hydrolysis so strongly favorable?
  2. Worked exampleFind how many ATP a cell must spend to drive a reaction needing 45 kilojoules per mole.
  3. Going furtherWhat changes when ATP hands its phosphate to a substrate instead of releasing it to water?
  4. Common mistakeWhy isn't ATP energetic because of some special bond chemists call a high-energy bond?
Your turnmedium1 of 3
A biosynthetic step requires 137 kilojoules per mole of free energy. Taking ATP hydrolysis to ADP and phosphate as minus 30.5 kilojoules per mole, how many ATP-equivalents of free energy is that?

1D. Biological Oxidation-Reduction and Electron Carriers

  1. How it worksHow does splitting a reaction into two half-reactions show exactly where the electrons go?
  2. How it worksWalk me through how NAD plus takes on two electrons and one proton to become NADH.
  3. Worked exampleFind how many electrons one NADH and one FADH2 each carry into the transport chain.
  4. Going furtherWhat changes about a carrier's job when it is NADPH headed for biosynthesis rather than NADH?
  5. Common mistakeWhy isn't oxidation always a matter of adding oxygen to something?
Your turneasy1 of 3
A mitochondrion receives 9 molecules of NADH and 6 molecules of FADH2. How many electrons do these carriers deliver to the electron transport chain in total?

1D. Carbohydrate Nomenclature, Configuration, Epimers and Anomers

  1. How it worksHow does an open-chain sugar close into a ring and create a brand new stereocenter?
  2. Going furtherWhat happens in solution as alpha and beta glucose interconvert through the open-chain form?
  3. Worked exampleFind which carbon separates glucose from mannose and which one separates it from galactose.
  4. Common mistakeWhy isn't an anomer the same kind of relationship as an epimer?
Your turnmedium1 of 3
An aldose has 7 carbon atoms in its open-chain form. How many stereoisomers of that aldose exist?

1D. The Glycosidic Linkage, Disaccharides and Polysaccharides

  1. How it worksHow does one flip from an alpha to a beta linkage turn a storage molecule into a structural one?
  2. Worked exampleFind which monosaccharides come out when maltose, sucrose and lactose are each hydrolyzed.
  3. Going furtherWhat changes about glycogen's release rate because of its alpha 1-6 branch points?
Your turneasy1 of 3
A linear starch chain of 363 glucose residues is completely hydrolyzed to free glucose. How many water molecules are consumed?

1D. Glycolysis: the Steps, the Substrates and the Net Yield

  1. How it worksHow does a six-carbon sugar get sliced into two three-carbon pieces halfway through glycolysis?
  2. Worked exampleFind the net ATP and NADH from one glucose, counting the investment phase and the payoff phase.
  3. Worked exampleFind which two glycolysis steps make ATP by substrate-level phosphorylation.
  4. Common mistakeWhy doesn't glycolysis need oxygen even though its product feeds an oxygen-hungry pathway?
  5. Going furtherWhat happens to the ATP count when the pathway starts from stored glycogen instead of free glucose?
Your turnmedium1 of 3
A muscle cell runs 45 glucose residues mobilized from glycogen as glucose 1-phosphate and 42 free glucose molecules through glycolysis. What is the total NET ATP yield?

1D. Regulation of Glycolysis: Hexokinase, PFK-1 and Pyruvate Kinase

  1. How it worksHow does ATP itself shut phosphofructokinase down once the cell already has plenty?
  2. How it worksWalk me through how fructose 2,6-bisphosphate overrides that brake when insulin is high.
  3. Going furtherWhat changes about control when the liver swaps hexokinase for glucokinase?
Your turnhard1 of 3
Liver glucokinase has a Michaelis constant for glucose of 10 millimolar. At a portal blood glucose concentration of 15 millimolar, what fraction of its maximum velocity does glucokinase run at?

1D. Fermentation and the Cori Cycle

  1. How it worksHow does reducing pyruvate to lactate regenerate the NAD plus that glycolysis is starving for?
  2. Common mistakeWhy isn't lactate the thing that makes muscles burn and ache two days later?
  3. Worked exampleFind the ATP cost to the liver of rebuilding one glucose from two lactate molecules.
  4. Going furtherWhat happens instead in yeast, where pyruvate becomes ethanol and carbon dioxide?
Your turnmedium1 of 3
The Cori cycle costs the liver 6 ATP equivalents for every glucose it rebuilds from two lactate molecules. How many ATP equivalents does the liver spend rebuilding 59 glucose molecules?

1D. Gluconeogenesis and the Bypass Reactions

  1. How it worksHow do four bypass enzymes step around the three irreversible reactions of glycolysis?
  2. Going furtherWhat happens to gluconeogenesis when glucagon climbs during an overnight fast?
  3. Common mistakeWhy isn't fatty acid carbon a usable source of new glucose in humans?
  4. Ties togetherHow do glycolysis and gluconeogenesis avoid running at once and burning ATP for nothing?
Your turnmedium1 of 3
Gluconeogenesis from pyruvate costs 4 ATP, 2 GTP and 2 NADH for each glucose made. Counting ATP and GTP together as nucleoside triphosphates, how many are spent making 43 glucose molecules?

1D. The Pentose Phosphate Pathway

  1. How it worksHow does the oxidative branch strip carbon dioxide off glucose and hand out two NADPH?
  2. Common mistakeWhy isn't NADPH interchangeable with NADH inside the cell?
  3. Going furtherWhat happens to a red blood cell under oxidative stress when G6PD activity is missing?
Your turneasy1 of 3
43 molecules of glucose 6-phosphate pass through the oxidative branch of the pentose phosphate pathway. How many NADPH are produced?

1D. Glycogen Synthesis, Breakdown and Their Control

  1. How it worksHow does glycogen phosphorylase clip glucose off an end without spending any ATP to do it?
  2. Going furtherWhat changes about glycogen handling in liver versus muscle when blood sugar falls?
  3. Worked exampleFind how many free ends a branched glycogen molecule offers compared with one unbranched chain.
  4. Common mistakeWhy isn't glycogen breakdown simply glycogen synthesis run in reverse?
Your turneasy1 of 3
A glycogen molecule carries 142 alpha 1,6 branch points. How many non-reducing ends does it offer to glycogen phosphorylase?

1D. Principles of Metabolic Regulation and the Dynamic Steady State

  1. How it worksHow does a dynamic steady state hold concentrations constant while material pours right through?
  2. Common mistakeWhy isn't a metabolic steady state the same thing as equilibrium?
  3. Going furtherWhat happens to flux through a pathway when you double an enzyme that was never limiting?
  4. Ties togetherHow do allosteric control, covalent modification and gene expression act on three timescales?
Your turnmedium1 of 3
A pathway at steady state carries a flux of 27 micromoles per minute, and the pool of one intermediate holds steady at 145 micromoles. What is the turnover time of that pool, in minutes?

1D. Pyruvate Dehydrogenase and Acetyl-CoA Production

  1. How it worksHow does a three-enzyme complex strip carbon dioxide off pyruvate and hand the rest to coenzyme A?
  2. Worked exampleFind how many NADH and carbon dioxide one glucose yields at the pyruvate dehydrogenase step.
  3. Going furtherWhat happens to the complex when acetyl-CoA and NADH pile up inside the mitochondrion?
Your turneasy1 of 3
29 glucose molecules are taken through glycolysis and on to acetyl-CoA. How many NADH are generated at the pyruvate dehydrogenase step alone?

1D. The Citric Acid Cycle: Reactions and Products

  1. How it worksHow do two carbons enter as acetyl-CoA and two leave as carbon dioxide on every turn?
  2. How it worksWalk me through where each NADH, FADH2 and GTP appears as the cycle goes around once.
  3. Worked exampleFind the total NADH, FADH2 and GTP from one glucose after two turns of the cycle.
  4. Common mistakeWhy doesn't the oxygen in the carbon dioxide you exhale come from the oxygen you breathed in?
  5. Ties togetherHow does the cycle double as a supply depot for amino acid and heme synthesis?
Your turneasy1 of 3
The citric acid cycle turns 29 times. How many NADH are produced?

1D. Regulation of the Citric Acid Cycle

  1. How it worksHow do NADH, ATP and calcium each tune isocitrate dehydrogenase up or down?
  2. Common mistakeWhy isn't the cycle controlled directly by how much oxygen happens to be present?
  3. Going furtherWhat happens when intermediates get pulled out for biosynthesis faster than they are replaced?
Your turnmedium1 of 3
Inhibition of isocitrate dehydrogenase cuts citric acid cycle flux to 62 percent of its previous value. If the cycle had been turning 62 times per second in a mitochondrion, what is the new rate, in turns per second?

1D. Beta-Oxidation of Saturated and Unsaturated Fats

  1. How it worksHow does each round of beta-oxidation clip exactly two carbons off the end of the chain?
  2. Worked exampleFind how many rounds and how many acetyl-CoA come from oxidizing 16-carbon palmitate.
  3. Worked exampleFind the NADH and FADH2 yield from palmitate before any of it reaches the citric acid cycle.
  4. Common mistakeWhy doesn't a long-chain fatty acid enter the mitochondrion without carnitine carrying it in?
  5. Going furtherWhat changes in the pathway when the chain has a double bond or an odd number of carbons?
Your turneasy1 of 3
Complete beta-oxidation of a saturated, even-chain fatty acid yields 10 molecules of acetyl-CoA. How many rounds of beta-oxidation were required?

1D. Digestion, Mobilization, Transport and Anabolism of Fats

  1. How it worksHow do bile salts and lipases break dietary fat into pieces small enough to absorb?
  2. How it worksWalk me through how chylomicrons, VLDL, LDL and HDL each move lipid somewhere different.
  3. Going furtherWhat happens to fatty acid synthesis when citrate spills into the cytosol after a big meal?
Your turneasy1 of 3
A meal delivers 66 grams of triacylglycerol into chylomicrons. At 9 kilocalories per gram, how much energy does that represent?

1D. Ketone Bodies

  1. How it worksHow does an overloaded liver turn surplus acetyl-CoA into acetoacetate and beta-hydroxybutyrate?
  2. Worked exampleFind how many acetyl-CoA a muscle recovers from one acetoacetate molecule.
  3. Going furtherWhat happens to blood pH when ketone production runs unchecked in uncontrolled diabetes?
Your turneasy1 of 3
43 molecules of acetoacetate are taken up by a muscle cell and converted back to acetyl-CoA. How many acetyl-CoA result?

1D. Metabolism of Proteins and the Fate of Amino Acid Nitrogen

  1. How it worksHow does transamination lift an amino group off one amino acid and set it on a keto acid?
  2. Worked exampleFind where the two nitrogen atoms in a single urea molecule originally came from.
  3. Going furtherWhat happens to the carbon skeleton left behind once the amino group has been stripped off?
  4. Common mistakeWhy isn't ammonia simply excreted directly instead of being built into urea first?
Your turnmedium1 of 3
Urea carries two nitrogen atoms per molecule, and nitrogen has an atomic mass of 14 grams per mole. A person excretes 16 grams of urea nitrogen in a day. How many moles of urea is that?

1D. The Electron Transport Chain

  1. How it worksHow does each complex pass electrons downhill and pump protons across the inner membrane?
  2. How it worksWalk me through where NADH and FADH2 each enter the chain and why their yields differ.
  3. Worked exampleFind how many protons get pumped per NADH compared with per FADH2.
  4. Going furtherWhat happens to every complex upstream the moment cyanide blocks complex IV?
  5. Common mistakeWhy isn't oxygen involved in any step of the chain except the very last one?
Your turnmedium1 of 3
29 molecules of NADH and 14 molecules of FADH2 are oxidized by the electron transport chain. Counting 10 protons pumped per NADH and 6 per FADH2, how many protons are pumped in all?

1D. Chemiosmotic Coupling, the Proton-Motive Force and ATP Synthase

  1. How it worksHow does the proton-motive force store energy as a charge difference and a pH difference at once?
  2. Worked exampleFind how many protons must rush through ATP synthase to make one molecule of ATP.
  3. Common mistakeWhy isn't ATP synthase simply the electron transport chain running backwards?
  4. Going furtherWhat happens to ATP output when the inner mitochondrial membrane springs a proton leak?
  5. Ties togetherHow does the rotating stalk of ATP synthase turn a gradient into three binding-site conformations?
Your turneasy1 of 3
ATP synthase requires about 4 protons per ATP, three through the rotor and one to import phosphate. How many protons must cross the inner membrane to make 73 ATP?

1D. The Net Energetic Result of Oxidizing One Glucose

  1. How it worksHow does the shuttle that carries cytosolic NADH inward change the final ATP count?
  2. Going furtherWhat changes about the total when the glycerol phosphate shuttle replaces the malate-aspartate one?
  3. Worked exampleFind the total ATP from one glucose using 2.5 per NADH and 1.5 per FADH2.
  4. Ties togetherHow do the ledgers from glycolysis, the cycle and the chain add up into one number?
Your turnmedium1 of 3
A liver cell completely oxidizes 22 glucose molecules, using the malate-aspartate shuttle to carry cytosolic NADH into the mitochondrion. Counting 2.5 ATP per NADH and 1.5 ATP per FADH2, what is the total ATP yield?

1D. Regulation of Oxidative Phosphorylation, Uncouplers and Oxidative Stress

  1. How it worksHow does respiratory control tie the electron chain's speed to the supply of ADP?
  2. Common mistakeWhy doesn't an uncoupler stop the electron transport chain from running?
  3. Going furtherWhat happens inside a brown fat cell when thermogenin opens a proton channel?
  4. Worked exampleFind what oxygen consumption and ATP output do when dinitrophenol is added to mitochondria.
  5. Ties togetherHow does a leaky mitochondrion connect reactive oxygen species to apoptosis?
Your turnmedium1 of 3
Isolated mitochondria consume oxygen at 309 nanomoles per minute with ADP present, and at 29 nanomoles per minute once the ADP has been used up. What is the respiratory control ratio?

1D. Insulin, Glucagon and the Fed and Fasted States

  1. How it worksHow does insulin push glucose into muscle and fat while telling the liver to store it?
  2. How it worksWalk me through what glucagon switches on in the liver over the first 12 hours of a fast.
  3. Common mistakeWhy isn't glucagon simply the exact opposite of insulin in every tissue?
  4. Worked exampleFind how blood glucose is defended hour by hour from the last meal out to a 3-day fast.
  5. Ties togetherHow does one hormone molecule reach dozens of enzymes through a single phosphorylation cascade?
Your turnmedium1 of 3
The liver holds 108 grams of glycogen, and the brain consumes about 6 grams of glucose per hour. Ignoring every other source and consumer, for how many hours could liver glycogen alone supply the brain?

1D. Tissue-Specific Metabolism and the Regulation of Body Mass

  1. How it worksHow do brain, muscle, liver, fat and red blood cells each settle on a different fuel?
  2. How it worksWalk me through how leptin and ghrelin report fat stores and hunger to the hypothalamus.
  3. Worked exampleFind the yearly weight gain from an extra 100 calories a day at 3500 calories per pound.
Your turneasy1 of 3
A person eats 224 kilocalories a day more than they burn. At 3500 kilocalories per pound of stored fat, how many pounds would they gain in a year?

AP®, SAT® and PSAT/NMSQT® are trademarks registered by the College Board (PSAT/NMSQT® jointly with the National Merit Scholarship Corporation). ACT® is a registered trademark of ACT Education Corp. GRE® is a registered trademark of ETS. LSAT® is a registered trademark of the Law School Admission Council. MCAT® is a registered trademark of the Association of American Medical Colleges. ISEE® and SSAT® are registered trademarks of their owners. None of these organizations is affiliated with, or endorses, Jentoo.